I. Field of the Invention
The present invention relates generally to computer architectures and, more particularly, to configuring the interconnection of circuit boards connected to a backplane of a microcomputer.
II. Related Art
Conventionally, microcomputer architectures are designed with a backplane to which printed circuit boards are connected. The backplane itself is a circuit board having external connectors and internal predetermined signal paths. In a sense, the backplane can be conceptualized as the main interface structure.
Circuit boards interfaced to the backplane can take many forms. The circuit boards can comprise a data processing device ("CPU board"), a data storage device ("memory board"), and/or a data input/output (I/O) device ("I/O board"), as well as interface logic and other electronic elements.
The circuit boards are interfaced to the backplane via slots. A slot is a position on the backplane where a circuit board can be physically inserted and thereby electrically connected to the signal paths on the backplane.
Oftentimes, microcomputer architectures require that a "daisy-chain" be maintained collectively among certain electronic lines of the circuit boards. A daisy-chain is a special type of signal line that connects the circuit boards in series, as opposed to in parallel. A daisy-chain is used to propagate a signal level from circuit board to circuit board in series, starting with a first slot in the backplane and ending with the last slot in the backplane. In contrast, a parallel connection simultaneously presents the signal to all circuit boards without allowing a board to intercept and hold the signal.
An example of a high performance industry standard bus which implements daisy-chains is the VMEbus, which was specified by IEEE standard 1014-87 based upon the VMEbus specification released by the VMEbus International Trade Association in August 1982. In this regard, see The VMEbus Specification, VMEbus International Trade Association (VITA), Document No. SH11544 (Mar. 28, 1988), which is incorporated herein by reference.
Specifically, the VMEbus has certain arbitration bus lines on its backplane. The arbitration bus lines comprise four daisy-chain lines, collectively referred to as "bus grant daisy-chains." In addition, the VMEbus requires one interrupt acknowledge daisy-chain on the backplane. Thus, the VMEbus comprises a total of five daisy-chain lines.
If a backplane slot is not occupied by a circuit board and if other circuit boards exist farther down the daisy-chain, then conventionally jumpers or some other bridge device must be installed at the empty slot to pass the daisy-chain signal through the slot opening and thereby maintain the continuity of the daisy-chain.
Some attempts have been made to implement an automatic mechanical switching device situated inside the backplane connectors. When a circuit board is inserted into the backplane, the mechanical switching device serves to maintain the daisy-chain in the backplane as usual. However, when a circuit board is not inserted into a backplane slot, then the mechanical switching device serves as a jumper across the pertinent connector, thereby maintaining the continuity of the daisy-chain.
The conventional methods for maintaining the daisy-chain lines in the absence of circuit boards are problematic. The need to introduce jumpers or other bridge devices across the circuit board connectors increases the complexity of assembly and does not provide for a user friendly computer. Moreover, the method utilizing the automatic mechanical switching device has been deemed unreliable and/or impractical in the industry. It is susceptible to mechanical jamming and other undesirable electrical malfunctions.
Consequently, a strong need exists in the industry for a reliable and automatic way of configuring various circuit boards in a computer architecture to thereby maintain the continuity of daisy-chain lines.